The present invention relates to a polyester film and a decorative plate using the polyester film. More particularly, the present invention relates to a polyester film for a decorative plate, which is free from environmental pollution upon burning such as incineration and capable of producing a picture pattern having a high design value thereon, and which has a high dimensional stability, a polyester film for a decorative sheet laminated by clear film, which is excellent in suitability for V-cutting process and resistance to impact deformation, a decorative plate or sheet using the polyester film, and a decorative plate or sheet laminated by clear film using the polyester film.
The decorative plate is in the form of a laminated sheet in which at least a film layer and a picture-printed layer are in turn superimposed over a surface of a substrate. For example, decorative plates in which an adhesive layer, a film layer and a picture-printed layer are in turn laminated over a surface of a substrate are called "printed decorative plates", and decorative plates in which an first adhesive layer, an inner film layer (A), a picture-printed layer, a second adhesive layer and an outer film layer (B) are in turn laminated over a surface of a substrate are called "decorative plates laminated by clear film".
The "decorative plate" used in the present invention generally means a building surface material capable of being dealt as an independent product. Whereas, a "decorative sheet" used in the present invention generally means a decorative surface material which is adapted to adhere to a surface of a substrate for various products such as furniture, building components or household appliances and provided at a surface thereof with a picture-printed layer. Accordingly, the film layer and the picture-printed layer laminated over the substrate of the decorative plate are regarded as components of the decorative sheet. Incidentally, as the substrates, wood-based boards such as plywood or particle boards, inorganic boards, steel boards or the like can be appropriately used.
Especially, the decorative plate laminated by clear film has such advantages that (i) an appropriate protection for the picture-printed layer can be attained by the outer film layer (B), (ii) when the outer film layer (B) is composed of a transparent film, the decorative plate can exhibit a high-grade picture pattern, or (iii) when the outer film layer (B) is composed of an embossed film, the decorative plate with a three-dimensional moire finishing can be produced. For these reasons, the decorative plates laminated by clear film can be used in higher-grade applications as compared with the printed decorative plates.
Owing to the afore-mentioned advantages, the decorative plates laminated by clear film can be applied to front face decorations of various furniture, doors or the like. In the case of furniture, doors or the like composed especially of a wood-based board, corners are defined by adjacent two flat portions which are crossed at a right angle. These corners can be formed by bending the afore-mentioned decorative plate.
The bending of the decorative plate has been generally performed by a so-called V-cutting process. In the V-cutting process, V-shaped notches are formed on a surface of a substrate of the decorative plate such that the depth of each notch is slightly smaller than the thickness of the substrate. The decorative plate is then bent at these V-shaped notches such that the notched surface of the decorative plate is located inside. For this reason, the suitability for denting deformation process, of the decorative plate is highly relied upon those of the films used therein. Especially, in the case of the decorative plates laminated by clear film in which inner and outer films are used, it is required that these films show more excellent suitability for denting deformation process as compared with in the case of the printed decorative plate having only one film. If the films are deteriorated in suitability for bending deformation process, the bent shape of the decorative plate cannot be kept stable when subjected to V-cutting and bending processes, but readily returns back to its original flat shape. Further, at this time, there is a likelihood that the decorative sheet composed of the inner film layer (A), the picture-printed layer, the adhesive layer and the outer film layer (B) is separated from the substrate, so that the decorative plate is deteriorated in its appearance or design value. Therefore, the films used in the decorative plate laminated by clear film is advisable to show excellent suitability for V-cutting and bending processes.
Also, in the case where the decorative plate is used as surface materials for furniture, doors or the like, it is inevitably brought into contact with, for example, tip ends of a cleaner, nails or the like. If the films adhered to the substrate of the decorative plate have low self-healing properties (restoring properties), dents caused thereon by the afore-mentioned contact are left as they are, so that a high design value of the picture pattern of the picture-printed layer cannot be maintained. In addition, if the films adhered to the substrate of the decorative plate do not have an appropriate strength or rigidity, plastic deformation thereof becomes increased and it is impossible to remove such dents by self-healing. Especially, in the case of the decorative plate laminated by clear film, it is advisable that the films to be adhered to the substrate of the decorative plate are excellent in self-recovery for dent deformation (resistance to impact deformation) which is a total property combining the afore-mentioned self-healing properties and strength or rigidity.
As a sheet material (base film) interposed between the substrate and the picture-printed layer of the decorative plate or used in the decorative sheet, vinyl-chloride resin sheets can be most generally used. However, in the case where the vinyl-chloride resin sheet is used, there arise such problems that a plasticizer blended in the sheet material is transferred to an adjacent adhesive layer, resulting in the deteriorated adhesion thereof, and that the vinyl-chloride resin sheet suffers from elongation or shrinkage upon heating due to low thermal dimensional stability thereof, thereby causing wrinkles thereon. Since the vinyl-chloride resin film is readily deformable, it shows a good suitability for the V-cutting and bending processes, but since the vinyl-chloride resin film is deteriorated in resistance to impact deformation, there arises another problem that the thickness of the film must be sufficiently increased upon practical use.
Moreover, in recent years, there have been serious problems that the vinyl-chloride sheet generates a chlorine gas upon burning such as incineration, thereby causing sources for acid rain or dioxine. In consequence, there is an increasing demand that decorative plates or decorative sheets should be produced without using the vinyl-chloride resin sheet from a standpoint of environmental protection.
Meanwhile, as described above, since decorative plates and decorative sheets are used as a surface material, a picture pattern having a high design value is generally formed on a surface thereof. Therefore, it is extremely important to adjust or control color tones of the decorative plates or decorative sheets in order to obtain appropriate contrasts between the picture patterns or delicate contrasts of shade and shadow thereon. As the substrates of the decorative plates or the substrates to which decorative sheets are attached, various materials such as plywood, e.g., particle boards, steel boards or the like have been used. In general, these substrates have a variety of color tones which are different from each other even if the substrates are composed of the same material.
Accordingly, if individual films for the decorative sheet has a deteriorated hiding power, the color tone of the substrate adversely affects that of the surface of the decorative plate or decorative sheet, so that an inherent high design value of the picture pattern formed therein is damaged. For this reason, it is required that the sheet materials or films for the decorative plates or decorative sheets have a high hiding power, thereby enabling these sheet materials or films to be widely applicable to substrates having various color tones.
Incidentally, with respect to decorative plates using other films than the vinyl-chloride resin films, there have been proposed a decorative plate in which a laminated material composed of a polyolefin-based synthetic paper as base film or a flexible polyester film as a base film is used (Japanese Patent Application Laid-open (KoKai) No. 7-17005 (1995)); a decorative plate in which a laminated material composed of a thermoplastic transparent elastomer film selected from the group consisting of styrene-based films, olefin-based films, urethane-based films, fluororubber-based films, polyamide-based films and ester-based films, or an polyolefin-based resin is used (Japanese Patent Application Laid-open (KoKai) No. 6-79830 (1995)); or the like.
However, the afore-mentioned conventional decorative plates using the polyolefin-based resin films are not suitable for furniture or interior building materials, because these decorative plates show a large calorific value upon burning, e.g., upon fire. For this reason, there is a demand for decorative plates using other films than the polyolefin-based resin films.
Biaxially-oriented polyester films, typically a polyethylene terephthalate film, have been widely utilized in various fields because of their excellent properties. However, as disclosed in Japanese Patent Application Laid-open (KoKai) No. 7-17005 (1995), these biaxially-oriented polyester films conventionally proposed are used only as surface materials for decorative plates. However, there has not yet been any proposal that the biaxially-oriented polyester film is interposed between the substrate and the picture-printed layer, in other words, that the biaxially-oriented polyester film is used as a sheet material (base film) for decorative sheets.
As a result of intense studies by the present inventors, it has been found that by using a biaxially-oriented polyester film having an optical density of 0.1 to 5.0 and longitudinal and transverse heat shrinkage percentages of not less than -10.0% and not more than +10.0% after heat-treated at 180.degree. C. for 5 minutes, the resulting decorative plate or sheet can generate no harmful gases upon burning such as incineration, can produce a picture pattern having a high design value thereon without adverse influence by the color tone of the substrate, and is excellent in dimensional stability. The present invention has been attained by the above finding.